The objectives of this study are primarily (1) to use thermoregulatory responses of the cat to evaluate the hypothesis that narcotic agents act on three or more distinct types of opioid receptor and (2) to determine the functional components of the thermoregulatory system primarily affected (a) by stimulation of each type of receptor and (b) by precipitation of abstinence in dependent animals. The results will be compared with effects produced by administration of endogenous opioid peptides or their analogs including enkephalins, endorphins, D-ala2-methionine-enkephalinamide, Fk 33, 824, etc. to determine if these peptides stimulate a specific type of receptor within the thermoregulatory system. Techniques will include (1) automatic recording of body temperature in unanesthetized, unrestrained cats, (2) drug injections primarily into the third cerebral ventricle via implanted cannulas, (3) visual observation for evidence of changes in physiological and behavioral thermoregulatory activities after drug injection, (4) determination of drug effects on body temperature at a variety of environmental temperatures, (5) assessment of the ability of opiate antagonists to alter drug and peptide-induced temperature changes and (6) heat-reinforcement and escape behavior in trained animals. Whenever possible, cross-over studies will be done in which each of the animals receives each of the possible treatments including control injections of equal volumes of vehicle in randomly determined order.